The present invention relates generally to apparatus for measuring the jitter of a signal and more particularly to an apparatus for measuring the jitter (time-axis fluctuation component) of a video signal which is reproduced by a video signal recording and/or reproducing apparatus.
In general, video signal recording and/or reproducing apparatus have differences in quality depending on the apparatus. There are certain irregularities in the rotating heads, fluctuations of type traveling speeds, and other deviations. When such irregularities, fluctuations, and deviations exist, jitter (a time-axis fluctuation component) occurs in the reproduced video signal. When jitter is present in a reproduced signal, the image on the picture screen of the television receiver sways in the horizontal direction. In general, the jitter is represented by a time unit (.mu. sec.) which has been converted in correspondence to the horizontal scanning time on a picture screen of a television receiver.
It is desirable that no jitter be produced in a video signal recording/reproducing apparatus. Therefore, jitter is measured in the final inspection process during the production of a video signal recording/reproducing apparatus.
However, while it is generally true that jitter exists in a reproduced signal, usually it is of only a very small magnitude. For this reason, it is almost impossible to inspect this magnitude with accuracy merely by supplying the reproduced signal to a television receiver and observing the resulting swaying of the image on the picture screen. Moreover, the automatic frequency control (AFC) characteristic of a commercial televison receiver, in general, differs with the manufacturer and the type of receiver. Even with the same jitter, the degree of swaying in the picture image differs with the television receiver.
Accordingly, heretofore, an apparatus for measuring jitter has been one wherein the jitter content of the reproduced signal is detected. An oscilloscope, a spectrum analyzer, or a pen recorder is directly operated in response to the resulting detection output. This method of detecting the jitter content of a signal is accomplished by detecting the time-axis fluctuation component of the horizontal synchronizing signal in the reproduced video signal. However, the horizontal synchronizing signal deviates greatly at the so-called switching point. That switching occurs at the time point of switching between the output signals of the rotating heads of a video signal recording/reproducing apparatus. For this reason, the jitter measurement is greatly influenced by this switching effect, whereby accurate measurement could not heretofore be carried out.
Furthermore, unless the operator of a known jitter measuring apparatus has considerable skill in performing the measuring operation, it is difficult to interpret of the resulting measurement data. Consequently, the determination of the magnitude of the jitter is difficult.